I saw an ad for 15/43 sprockets & 520 chain that said this mod is good for .7 in the quarter mile--that sounds a bit unbelievable. What amount of top end will be given up and what is a realistic 1/4-mile figure with 15/43.
YTKinWI?

 

I don't know about the quarter mile time because there are so many variables. You won't give up anything on top end because with stock gearing the Super Hawk will go faster in 5th than in 6th. With 15/43 it will pull to redline in 6th, keeping ultimate top speed about the same as stock.
The most noticeable change will be the way bike pulls in the midrange in any gear. It just makes it much more responsive at the expense of freeway buzziness. I have 15/43 and won't be going back to stock. Others don't like it as much.

 

I think 16/41 is stock, so down 1 on the front would be about equal to 2 more on the back, so that's about 4 more on the back in total -

45/41 = 1.1, so expect engine speed to be about 10% higher at whatever road speed than before.

Should make 2nd gear roll-on mono's a bit easier to do.

 

I went up 2 on the rear sprocket just because I was never using 6 on the highway and it was almost lugging to do so. this is better. I would next go lower on the front because I still rarely get out of 4th and 5th on a highway ride. Go stepwise and see what you like . in your normal riding if you are never using the higher gears you have a lot of room to go.

 

When I did my build in 07 I could get the nuts loose on the rear so I installed the 15T front only. Stock is 16/41. My config is "jerk it out of your hands" and it's an easy wheelie machine through second. The kid can power on wheelie in low and ride it to "grease" the front down with ease while still accelerating.

I like the road with this gearing. It's got a little more lower than the 15/43 but if you're in "runaway mode" you can be gone in a flash. Last time TxSuperChicken and I rode we traded bikes in Bastrop and ran 30 miles. He was running stock gears at the time but bought a new front sprocket before heading for home that afternoon.

I have NIB rears from a member here in both 43 and 41. When my current chain goes I'm going to the 520. That means I'll have to get a new front in 520 but it's still a toss-up for me whether I'll mount the 41 or the 43.